Vacuum means: A space entirely devoid of matter (called also, by way of
distinction, absolute vacuum); hence, in a more general sense, a space,
as the interior of a closed vessel, which has been exhausted to a high
or the highest degree by an air pump or other artificial means; as,
water boils at a reduced temperature in a vacuum.

Vacuum means: The condition of rarefaction, or reduction of pressure
below that of the atmosphere, in a vessel, as the condenser of a steam
engine, which is nearly exhausted of air or steam, etc.; as, a vacuum
of 26 inches of mercury, or 13 pounds per square inch.